Mobile communications networks are currently evolving from circuit switched (CS) networks towards packet switched (PS) networks, and by that integrate into Internet Protocol (IP) based infrastructures that are e.g. used by the Internet and the World Wide Web respectively.
More recently, an architecture called IP Multimedia Subsystem (IMS) has been defined by the Third Generation Partnership Project (3GPP) for delivering multimedia services over mobile communication networks. IMS is able to provide new and rich person-to-person communication services or network-to-person communication services by means of text, audio, video, and/or messaging or any combination thereof, employing IP for transport and Session Initiation Protocol (SIP) for service signaling. Hereto, calls from and to subscribers of the multimedia services using a Circuit Switched based access (e.g. WCDMA/GERAN), in the following also being referred to as CS access, are routed through the IMS network in order to reach an IMS service engine. This concept is called IMS Centralized Services (ICS), and is described in the standardization documents TS 23.237 (this application referring to release 11.3.0), TS 23.292 (this application referring to release 11.1.0), and 3GPP TS 23.228 (this application referring to release 11.3.0) established by the 3rd Generation Partnership Project—3GPP—.
Terminating Access Domain Selection T-ADS according to TS 23.292, section 5.3.1 provides:                Directs an incoming session to an ICS User;        For one or more UEs of an ICS User:                    Influences the selection of one or more contacts amongst the registered contacts and;            Influences the selection of an access network for delivery of the incoming session to the selected contact, or;            Performs breakout to the CS Domain by fetching the CSRN.                        
T-ADS shall take into account the access network's capabilities, UE capabilities, IMS registration status, CS status, existing active sessions, user preferences, operator policies such as access network specific voice domain preferences and the media component types. In addition, T-ADS may take into account the access network information provided directly by the UE.
The current procedures for T-ADS as described in current 3GPP documents might not provide for allowing a best effort packet switched access at choice of the user. The user might prefer a certain packet switched access even in cases where other packet or circuit switched access is available at presumably better quality. Such possibility of choice might be especially preferable e.g. in case that the subscriber has, besides the first line telephony service, also a second-line voice (or voice and video call) service that he or she prefers to use in certain areas or situations, e.g. when roaming, or in a hotel.